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Stock-based Compensation | 3 Months Ended
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Mar. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Stock-based Compensation |
7. | STOCK-BASED COMPENSATION
Restricted Stock Unit Awards Along with cash-based salaries and performance-based annual cash incentive awards, restricted stock unit awards represent an important element of compensation to our employees. Restricted stock unit activity is presented in the following tables.
| Shares (thousands) |  | Weighted Average Grant Date Fair Value | Weighted Average Remaining Contractual Life (in years)
Unvested as of December 31, 2017 | 727.7 |  |  | $ | 118.96 |  | 1.24
Granted | 119.3 |  |  | 160.86 |  |
Vested | (173.8 | ) |  | 121.15 |  |
Forfeited | (15.2 | ) |  | 124.62 |  |
Unvested as of March 31, 2018 | 658.0 |  |  | $ | 126.31 |  | 2.20
Unvested as of December 31, 2016 | 750.9 |  |  | $ | 113.97 |  | 1.71
Granted | 140.9 |  |  | 116.42 |  |
Vested | (103.2 | ) |  | 89.44 |  |
Forfeited | (2.9 | ) |  | 115.93 |  |
Unvested as of March 31, 2017 | 785.7 |  |  | $ | 117.62 |  | 1.82
We determine the fair value of restricted stock units based on the closing market price of the Company's common stock on the grant date. As of March 31, 2018, we had $33.6 million of unamortized deferred compensation related to unvested restricted stock units, which we anticipate recognizing over varying periods into 2022.

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- Definition The entire disclosure for compensation-related costs for equity-based compensation, which may include disclosure of policies, compensation plan details, allocation of equity compensation, incentive distributions, equity-based arrangements to obtain goods and services, deferred compensation arrangements, employee stock ownership plan details and employee stock purchase plan details. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -URI http://asc.fasb.org/subtopic&trid=2208855 + Details
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